Preamble allocation method and random access method in mobile communication system

ABSTRACT

The present invention relates to a preamble allocation method and a random access method in a mobile communication system. In the present invention, one among preamble resources is allocated to specific mobile stations in advance, before random access. The mobile stations request random access by transmitting a preamble based on the pre-allocated preamble resource to a base station. It is therefore possible to prevent collision caused by the fact that other mobile stations transmit the same preamble.

TECHNICAL FIELD

The present invention relates to a resource allocation method in amobile communication system. More particularly, the present inventionrelates to a method for allocating a preamble resource and a method forperforming random access using the preamble resource.

BACKGROUND ART

A base station and a mobile station perform a random access procedure toset up an initial call in a circuit-based mobile communication systemincluding WCDMA (wideband code division multiple access).

The random access procedure starts when a mobile station that attemptsto access a base station transmits a signal sequence having a specificpattern. The base station manages the signal sequence information havinga specific pattern (hereinafter called a “preamble”) based on contentionso that any mobile station can use it. Preambles are designed to bedetected easily without pre-information. For this purpose, indexes maybe used to identify preamble patterns. Information on these preambles ispre-fixed by a system and then used, or is obtained from systeminformation broadcasted from a base station.

In order to perform the random access procedure, the mobile stationselects a preamble pattern (or an index) from among information onpreambles, generates a preamble based on the selected preamble pattern,and transmits the same to the base station based on contention. Thepreamble is transmitted during an access slot interval having apredetermined length, and the mobile station transmits the selectedpreamble pattern during an initial interval of the access slot interval.

The base station transmits response information using a pattern or anindex of a corresponding preamble to the mobile station when thepreamble is detected within a specific interval, and then a call set-upprocedure is initiated. The random access procedure is performed by amobile station that is in a state (e.g., idle state) of having noconnection with the base station.

The random access procedure is performed as described above in a packetswitch-based mobile communication system.

However, in the packet switch-based mobile communication system, even ifa mobile station is in an active state in which packet service isprovided between the mobile station and the base station, a radioresource for packet information communication is not dedicatedlyallocated to the mobile station while the packet service is provided.That is, logical channels continue to exist so as to transmit packetdata, while physical channels, that is, radio resources, are shared by aplurality of mobile stations that are located within service coverage ofthe base station.

Accordingly, the mobile station should perform the random accessprocedure to obtain the uplink radio resource when uplinksynchronization is not maintained even though the mobile station is inan RRC (radio resource control)-connected state with the base station.

However, the random access procedure is contention-based, and therebycollision occurs. As a result, it causes delay in performing the randomaccess procedure. Further, it causes delay in allocating radioresources.

DISCLOSURE Technical Problem

The present invention has been made in an effort to provide a method forefficiently allocating preamble resources in order to reduce thepossibility of delay or collision in a random access procedure.

The present invention has been made in another effort to provide amethod for performing a random access procedure based on the preambleresources that are managed by the preamble resource allocation method.

Technical Solution

According to a first aspect of the present invention, a method forallocating a preamble includes dividing preamble resources into acontention-based group and a reserved allocation group, and allocating apreamble resource of the reserved allocation group to a mobile station.

According to a second aspect of the present invention, a method forallocating preamble includes allocating one of preamble resources forconstituting a preamble to a mobile station, and dividing preamble radioresources for preamble transmission into a contention-based region and areserved allocation region in which the mobile station receiving theallocation of the preamble resource transmits a preamble.

In the preamble allocation method, when the preamble includes a firstpart for representing a random property and a second part forrepresenting additional information, the step of dividing may includedividing the preamble resources into a contention-based region, areserved allocation region, and an additional information region.

In this case, in the mobile communication system, a preamble resource ofthe additional information group is allocated to a mobile station thatsatisfies one of predetermined conditions for preamble allocation, and apreamble resource of the reserved allocation group is allocated to amobile station that satisfies the remaining predetermined conditions.

According to a third aspect of the present invention, a method for amobile station to perform random access in a mobile communication systemincludes receiving a preamble resource of a plurality of preambleresources from a base station, generating a preamble based on thepreamble resource, requesting random access by transmitting the preambleto the base station, and receiving a response to the random accessrequest and acquiring synchronization with the base station according tothe response.

In the random access method, preamble resources are divided into acontention-based group and a reserved allocation group, and the step ofreceiving the preamble resource may include receiving a preambleresource of the reserved allocation group from the base station.Preamble radio resources for preamble transmission are divided into acontention-based region and a reserved allocation region, wherein thestep of requesting random access transmits the preamble to the basestation through the reserved allocation region.

According to a fourth aspect of the present invention, a method for amobile station to perform random access in a handover process in amobile communication system includes receiving a message provided from atarget base station in response to a handover request from a source basestation, the message including a preamble resource that is selected bythe target base station from a plurality of preamble resources,generating a preamble based on the preamble resource, requesting randomaccess by transmitting the preamble to the target base station, andreceiving a response to the random access request and acquiringsynchronization with the target base station according to the response.

ADVANTAGEOUS EFFECTS

It is possible to prevent collision caused by contention from a randomaccess procedure and to reduce time delay in the procedure since apreamble resource for random access is allocated to a mobile station inadvance, and the mobile station uses the allocated preamble resource inperforming random access in mobile communication.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a configuration diagram of a mobile communication systemaccording to an exemplary embodiment of the present invention.

FIG. 2 shows a brief flowchart of a random access method according tothe exemplary embodiment of the present invention.

FIG. 3 shows a preamble allocation method according to a first exemplaryembodiment of the present invention.

FIG. 4 shows a flowchart of a random access method according to a firstexample of the first exemplary embodiment of the present invention.

FIG. 5 shows a flowchart of a random access method according to a secondexample of the first exemplary embodiment of the present invention.

FIG. 6 shows a preamble allocation method according to a secondexemplary embodiment of the present invention.

FIG. 7 shows an example of a preamble allocation method according to athird exemplary embodiment of the present invention.

FIG. 8 shows another example of a preamble allocation method accordingto the third exemplary embodiment of the present invention.

MODE FOR INVENTION

In the following detailed description, only certain exemplaryembodiments of the present invention have been shown and described,simply by way of illustration. As those skilled in the art wouldrealize, the described embodiments may be modified in various differentways, all without departing from the spirit or scope of the presentinvention. Accordingly, the drawings and description are to be regardedas illustrative in nature and not restrictive. Like reference numeralsdesignate like elements throughout the specification. Throughout thespecification, unless explicitly described to the contrary, the word“comprise” and variations such as “comprises” or “comprising” will beunderstood to imply the inclusion of stated elements but not theexclusion of any other elements.

In this specification, a mobile station (MS) may refer to a terminal, amobile terminal (MT), a subscriber station (SS), a portable subscriberstation (PSS), user equipment (UE), or an access terminal (AT). Themobile terminal may include all or part of the functions of the mobilestation, the subscriber station, the portable subscriber station, andthe user equipment. In this specification, a base station (BS) may referto an access point (AP), a radio access station (RAS), a node B, a basetransceiver station (BTS), or an MMR (mobile multihop relay)-BS. Thebase station may include all or part of the functions of the accesspoint, the radio access station, the node B, the base transceiverstation, and the MMR-BS.

Now, an exemplary embodiment of the present invention will be describedin detail with reference to the accompanying drawings.

FIG. 1 is a configuration diagram of a mobile communication systemaccording to an exemplary embodiment of the present invention.

As shown in FIG. 1, in the mobile communication system according to theexemplary embodiment of the present invention, a base station 100communicates with a plurality of mobile stations (201, 202, . . . 20 n,where n is a positive integer) through a radio channel environment 300.For the better comprehension and ease of description, a mobile stationwill be assigned the representative number 200.

The base station 100 communicates with the mobile station 200 andperforms radio resource allocation for an up-link channel of the mobilestation 200 on the basis of information from the mobile station 200.

The communication system including the OFDMA (orthogonal frequencydivision multiplexing access) system according to the exemplaryembodiment of the present invention uses radio resources of atwo-dimensional structure with frequency and time. The radio resourcesinclude radio resource blocks that are divided into a TTI (transmissiontime interval), that is, a transmission interval, and a sub-carriergroup. Radio frames that constitute radio resources include TTI (or aslot) with a specific size (e.g., 0.5 msec). For example, a wirelessframe of 10 msec includes 20 slots. The radio resource is divided intodownlink radio resources and uplink radio resources, and the uplinkradio resources include radio resources for preamble transmission ofmobile stations. For better comprehension and ease of description, theradio resource for the preamble transmission will be referred to as“preamble radio resource”.

The preamble radio resource may be situated in a fixed slot (e.g., afirst slot or a last slot) of an uplink wireless frame, and a preamblemay be transmitted by random access burst units through the preambleradio resource.

Preambles may be generated by various methods. It is possible toconstitute a preamble set having a specific pattern, for example togenerate a maximum of 64 preambles with different patterns. Each patternis identified with an index, and the index is referred to as a “preambleindex”. The preamble index may be represented with 6 bits. A mobilestation randomly selects one from among 64 preamble indexes that areeach represented with 6 bits when performing random access, generates apreamble based on the selected preamble index, and transmits the same.In the exemplary embodiment of the present invention, the preambleresources include all of things that constitute a preamble, such as apreamble pattern and a preamble index.

In the mobile communication system having these features, a mobilestation maintains uplink physical layer synchronization with a basestation so as to transmit information. That is, the base stationcontrols timing of uplink signals transmitted by the mobile station sothat the signals are received within a CP (cyclic prefix) interval thatis determined in advance. For this purpose, the mobile station performsrandom access, thereby setting synchronization with the base station andrequesting radio resources.

The mobile station performs random access to access the base stationwhen uplink physical layer synchronization with the base station is notset or the mobile station is in an idle state. In addition, the mobilestation performs random access to request a radio resource in an activestate (or a connected state) when the mobile station communicates with abase station or in a state when uplink physical layer synchronizationwith the base station is maintained.

In the exemplary embodiment of the present invention, one among preambleresources is allocated to a mobile station before random access, whichstarts in variable states, so that contention between mobile stations isprevented and time for processing the random access procedure isreduced. As a result, the mobile station transmits a preamble on thebasis of the pre-allocated preamble resource and requests random access.Particularly, in the exemplary embodiment of the present invention, amobile station that satisfies conditions receives a preamble resource,that is, a preamble index, in advance. The conditions include thefollowing.

1) A condition that a mobile station has not exchanged packetinformation with a base station for more than a predetermined time or amobile station does not maintain uplink synchronization because ofvarious reasons in the following conditions.

1-1) In the condition that a mobile station receives information beingtransmitted from a base station and notifying of downlink servicerestart, and performs random access to achieve uplink physical layersynchronization.

1-2) In the condition that a base station allocates a radio resource toa mobile station for status reporting or measurement reporting and thenthe mobile station performs random access to achieve uplink physicallayer synchronization.

2) In the condition that a mobile station performs random access tostart packet transmission to a target base station in a handoverprocess.

In the exemplary embodiment of the present invention, a preamble indexis allocated to a mobile station that satisfies the above conditions(these are referred to as “predetermined conditions” for bettercomprehension and ease of description) in advance. The predeterminedconditions according to the exemplary embodiment of the presentinvention are not restricted to the above conditions.

FIG. 2 shows a flowchart of a random access method according to anexemplary embodiment of the present invention.

The random access method according to the present invention is generallydescribed below. As shown in FIG. 2, a base station 100 broadcastsinformation on preambles to mobile stations that are positioned in aservice area (S100).

When a mobile station 200 satisfies the predetermined conditions, thebase station 100 selects one from among preamble indexes and providesthe same to the mobile station 200 (S110). The mobile station 200requests random access based on the selected preamble index, and therebyprevents contention between mobile stations (S120). The random accessmethod will be described in detail below.

However, mobile stations that are not provided with a preamble index inadvance randomly select one from among preamble indexes and performcontention-based random access.

The preamble allocation method and the random access method on the basisof the preamble allocation method according to exemplary embodiments ofthe present invention based on the above features of the presentinvention will be described in detail below.

First, the preamble allocation method and the random access method onthe basis of the preamble allocation method according to a firstexemplary embodiment of the present invention will be described.

FIG. 3 shows the preamble allocation method according to the firstexemplary embodiment of the present invention.

In the first exemplary embodiment of the present invention, some of theentire preamble indexes that can be generated from preamble resourcesfor preamble transmission for random access of uplink are used forreserved-based random access, and the remaining are used forcontention-based random access.

As shown in FIG. 3, radio resource blocks, which are made withsubcarrier indexes and correspond to the TTI transmission interval whichis a symbol index, are allocated as preamble radio resources. Some ofthe entire preamble indexes that are constructible from the preambleresources are classified to a reserved allocation group G1, and theremaining are classified to a contention-based group G2.

Radio resource allocation information on preamble radio resourcesincludes radio resource block position information and radio resourceallocation interval information. The radio resource block positioninformation is to address radio resources identified with a frequencyaxis and a time axis, and includes subcarrier indexes and symbolindexes.

The radio resource allocation interval information represents in whichinterval an allocated radio resource is valid. The radio resourceallocation information on an allocated preamble radio resource may bepredetermined and is then applied in the entire system as the same.Also, the radio resource allocation information may be included insystem information and then broadcasted through a broadcasting channelby a base station. Furthermore, the radio resource allocationinformation may be transmitted using a radio resource that is allocatedfor downlink scheduling information transmission.

The random access according to the first exemplary embodiment of thepresent invention will be described based on preambles that are managedby the preamble allocation method.

A random access method according to a first example of the firstexemplary embodiment of the present invention will be described.

FIG. 4 shows a flowchart of the random access method according to thefirst example of the first exemplary embodiment of the presentinvention.

The random access method according to the first example of the firstexemplary embodiment of the present invention is performed when packetinformation has not been transmitted/received for more than apredetermined time in an active state in which a mobile stationcommunicates with a base station, or when downlink data to betransmitted to a mobile station that does not maintain uplinksynchronization with a base station for some reason is generated.

As shown in FIG. 4, a base station 100 generates downlink data andindication information to a corresponding mobile station when downlinkdata to be provided to a mobile station 200 that is in the above statesis generated (S200-S230).

Particularly, the base station 100 selects one from among the preambleindexes of the reserved allocation group G1, includes it in the downlinkdata generating indication information, and transmits the same through adownlink radio resource (S240).

The mobile station 200 receives the downlink data generating indicationinformation, extracts the preamble index therefrom, and generates apreamble based on the preamble index (S250-S260). The mobile station 200requests random access by transmitting the generated preamble to thebase station 100 (S270). The mobile station 100 transmits the preambleon the basis of the preamble index that is allocated by the base stationin advance, as described above. As a result, collisions because othermobile stations use the same preamble are prevented, and so a delay isprevented.

In the above preamble transmission step, the mobile station 200transmits the generated preamble through a preamble radio resource (PR)(referring to FIG. 2). The mobile station may obtain resource allocationinformation on the preamble radio resource from system informationbroadcasted through a broadcasting channel. Also, the resourceallocation information may be included in the generated downlink dataindication information from the base station.

When receiving the preamble, the base station 100 generates a responsemessage to the random access request according to whether or not thepreamble index included in the preamble is equal to the preamble indexthat is allocated to the mobile station in the steps of S220-S230, andtransmits the response message to the mobile station 200 (S280). Theresponse message includes a positive response (ACK) or a negativeresponse (NACK), representing whether or not the base station receivesthe preamble from the mobile station that requests random access.Further, downlink data allocation information on radio resources inwhich downlink data will be transmitted, a scheduling indicator, and soon may be included in the response message.

Therefore, the mobile station 200 retransmits the preamble when NACK isincluded in the response message, whereas the mobile station 200receives downlink data to be transmitted through a radio resourcecorresponding to the downlink radio resource allocation informationlater when ACK is included in the response message S290.

The random access according to a second example of the first exemplaryembodiment of the present invention will now be described.

FIG. 5 shows a flowchart of the random access method according to thesecond example of the first exemplary embodiment of the presentinvention.

The random access method according to the second example of the firstexemplary embodiment of the present invention is performed when a mobilestation wants to perform a handover.

As shown in FIG. 5, a base station 100 performs a general process for ahandover when receiving a handover request message for a mobile stationfrom a neighboring base station 10 n, and then selects one from amongthe preamble indexes of the reserved allocation group G1 (S310). Thebase station 100 includes the selected preamble index in a responsemessage to the handover request, and transmits the same to theneighboring base station 10 n (S320-S330).

The mobile station 200 extracts a preamble index that is allocated bythe base station 100, that is, a target base station, from the responsemessage sent by the neighboring base station 10 n, that is, a sourcebase station (S340-S350).

When the handover is completed, the mobile station 200 transmits apreamble that is generated based on the extracted preamble index to thetarget base station 100, requests random access (S360), and performsdata transmitting/receiving with the target base station 100 through aradio resource that is allocated by the target base station 100according to a random access response message (S370-S390).

In this case, the base station 100 may transmit resource allocationinformation on a preamble radio resource by including it in the responsemessage to the random access request, and the mobile station 200 maytransmit the preamble through the preamble radio resource (PR).

Accordingly, the mobile station to perform the handover transmits thepreamble based on the preamble index that is allocated in advance,thereby preventing a collision from occurring when other mobile stationsuse the same preamble and processing delay.

The random access method according to the second example of the firstexemplary embodiment of the present invention is performed in a backwardhandover in which a mobile station requests a target base station toperform a handover via a source base station. In addition, random accessusing the preamble indexes of the contention-based group in a forwardhandover in which a mobile station directly requests a target basestation to perform a handover may also occur. However, the presentinvention is not restricted to the above-described situations.

A base station may allocate preamble indexes of the reserved allocationgroup to a mobile station as above-described when requesting a mobilestation that does not maintain uplink synchronization in an activestatus to perform status reporting or measurement reporting so that themobile station performs random access on the basis of the allocatedpreamble index. In addition, the base station may transmit the preambleindex by including it in information on uplink radio resources forstatus reporting or measurement reporting. Since a person of ordinaryskill in the art can realize the random access method of the mobilestation based on the examples according to the exemplary embodiment ofthe present invention, a detailed description of the method is omitted.

Meanwhile, contrary to the first exemplary embodiment, when a mobilestation does not receive a preamble index in advance, the mobile stationrandomly selects one from among contention-based preamble indexes andperforms contention-based random access by transmitting a preamble basedon the selected preamble index through the preamble radio resource. Sinethis random access method is disclosed in the art, a detaileddescription of the method is omitted.

Next, a preamble allocation method and a random access method based onthe preamble allocation method according to a second exemplaryembodiment of the present invention will be described.

In the second exemplary embodiment of the present invention, preambleradio resources are divided into a contention-based region PR1 and areserved allocation region PR2 so that a mobile station that receives apreamble index in advance transmits a preamble through the reservedallocation region PR2 and a mobile station that does not receive apreamble index in advance transmits a preamble through thecontention-based region PR1. In this case, a specific preamble index isallocated to a mobile station that satisfies the predetermined conditionin advance.

FIG. 6 shows a flowchart of the preamble allocation method according tothe second exemplary embodiment of the present invention.

As shown in FIG. 6, the preamble radio resources PR in radio resourcesthat are identified with a frequency axis and a time axis are dividedinto the contention-based region PR1 and the reserved allocation regionPR2. The contention-based region PR1 includes preamble radio resourcescorresponding to contention-based preamble indexes, and the reservedallocation region PR2 includes preamble radio resources corresponding toreserved allocation preamble indexes, that is, preamble indexes that areallocated by a base station in advance.

In the second exemplary embodiment of the present invention in which thepreamble radio resource allocation is performed, mobile stations requestrandom access by transmitting a preamble based on a preamble index tothe base station through the reserved allocation region PR2 of thepreamble radio resources when the mobile station receives the preambleindex in advance from the base station through the downlink datagenerating indication information or the handover response message asdescribed in the first exemplary embodiment.

Therefore, the base station 100 can determine which mobile stationtransmits the preamble when receiving the preamble through the reservedallocation region PR2. In this case, it is unnecessary to perform acontention resolution procedure since there is no contention betweenmobile stations. As a result, it is possible to prevent a delay causedby contention from processing random access.

In the second exemplary embodiment, the base station may select one fromamong the entire preamble indexes and allocate it to a mobile stationthat satisfies the predetermined condition in advance. Also, the entirepreamble indexes may be divided into a reserved allocation group G1 anda contention-based group G2 as described in the first exemplaryembodiment. In this case, one among the preamble indexes of the reservedallocation group G1 is selected and allocated to a mobile station thatsatisfies the predetermined condition. The mobile station may transmit apreamble generated on the basis of the allocated preamble of thereserved allocation group G1 through the reserved allocation region PR2of the preamble radio resources (PR).

Since a person of ordinary skill in the art can realize the randomaccess method according to the second exemplary embodiment of thepresent invention based on the first exemplary embodiment, a detaileddescription of the random access method is omitted.

Meanwhile, a mobile station randomly selects one from among preambleindexes and performs random access by transmitting a preamble generatedon the basis of the selected preamble index through the contention-basedregion PR1 of the preamble radio resources when not receiving a preambleindex in advance from a base station. In addition, when the preambleindexes are divided as in the first exemplary embodiment, the mobilestation randomly selects one from among the preamble index of thecontention-based group G2 and transmits a corresponding preamble throughthe contention-based region PR1.

Next, a preamble allocation method and a random access method based onthe preamble allocation method according to a third exemplary embodimentof the present invention will be described.

The third exemplary embodiment of the present invention relates to amethod of managing preambles that include additional information, and amethod for a mobile station to perform random access according to thepreambles.

Additional information may be included in a preamble and transmitted ina random access procedure for efficiency of resource allocation. Thatis, a preamble that includes a first part representing a random propertyand a second part representing additional information may betransmitted. For example, when the preamble is represented with 6 bits,the random property is 5 bits and the additional information is 1 bit.

The following may be considered as additional information that may beincluded in the preamble.

1) CQI (channel quality indicator) information that may represent pathloss of a mobile station.

2) Information representing a reason for a mobile station to attempt toperform random access, which may include the following.

Information representing that a mobile station is performing a handover.

Information representing that the present access attempt is not aninitial access to a base station.

Information representing that uplink synchronization acquisition orresource allocation is requested.

When the additional information is 1 bit, it may be difficult torepresent the above information with 1 bit. In this case, the followingmay be included as additional information.

1) CQI information: represents that a corresponding channel has qualityof more than a predetermined value in a service area of a base station.

2) Information representing that a mobile station accesses via aneighboring base station in a handover process, which will be referredto as “handover indication information”.

3) Information representing that a mobile station does not performinitial access and maintains a connection with a base station, whichwill be referred to as “active state indication information”.

A base station may perform various functions based on the additionalinformation that is transmitted and included in a preamble. For example,the base station can adaptively allocate a radio resource based on CQIinformation that is included in a preamble transmitted from a mobilestation when transmitting a response message according to random access.

Considering the case in which a preamble including additionalinformation is transmitted as described above, preamble indexes can bedivided into a contention-based group, a reserved allocation group, andan additional information group. Preamble indexes of the additionalinformation group are allocated to a mobile station corresponding toadditional information to be included in a preamble. Preamble indexes ofthe reserved allocation group are allocated to a mobile station thatdoes not correspond to the additional information but satisfies thepredetermined conditions as in the first exemplary embodiment. Preambleindexes of the contention-based group are allocated to other mobilestations.

When a preamble index for random access includes a random property of afirst bit (e.g., 5 bit) and additional information of a second bit(e.g., 1 bit) for an upper layer (the upper layer of a physical layer),two cases are considered as follows.

A first case in which additional information represents a mobile stationperforming a handover process via a neighboring base station will befirstly described.

FIG. 7 shows a preamble allocation method according to the thirdexemplary embodiment of the present invention corresponding to the firstcase.

It is assumed that there are preamble indexes of “000000”-“111111” asshown in FIG. 7.

Preamble indexes of “100000”-“111111” are set as an additionalinformation group. Preamble indexes of an additional information groupG3 are for mobile stations transmitting a preamble in which additionalinformation of 1 bit represents handover indication information.

Other preamble indexes except for the additional information group G3are divided into the reserved allocation group G1 and thecontention-based group G2. In this instance, the preamble indexes of thereserved allocation group G1 are allocated to the remaining mobilestations that satisfy the predetermined condition from which mobilestations performing handover are excluded. That is, since preambleindexes for mobile stations performing handover are preset as theadditional information group G3, the preamble index of the reservedallocation group G1 is allocated to mobile stations to which a basestation transmits downlink data generating starting information.

More particularly, a base station 100 selects a preamble index from theadditional information group G3, includes it in a handover responsemessage, and transmits the same, as shown in the second example of thefirst exemplary embodiment, when receiving a handover request from aneighboring base station so that a corresponding mobile stationtransmits a preamble based on the preamble index.

However, even if the mobile station is performing handover, when themobile station directly requests a handover without passing through theneighboring base station, that is, a source base station, the basestation uses a preamble index of the contention-based group G2. Thepreamble indexes of the additional information group G3 may also bemanaged for mobile stations performing a handover based on contention.In this case, the preamble indexes of the reserved allocation group G1may be only allocated to mobile stations satisfying the predeterminedcondition as in the first exemplary embodiment.

According to the management of the preamble indexes, when a preambleincludes additional information representing that a corresponding mobilestation is accessing through a handover process from a neighboring basestation, a base station can easily determine whether or not thecorresponding mobile station includes a preamble index that ispre-allocated to mobile stations that access through a handover processon the basis of the additional information and the index of thepreamble.

A second case in which additional information represents a mobilestation that does not perform initial access but rather maintainsconnection with a base station will now be described.

That is, the additional information represents that a correspondingmobile station receives allocation of its own identifier (or ascheduling identifier) from the base station.

FIG. 8 shows a preamble allocation method according to the thirdexemplary embodiment of the present invention corresponding to thesecond case.

It is assumed that there are preamble indexes of “000000”-“111111” asshown in FIG. 8.

Preamble indexes of “100000”-“111111” from the entire preamble indexesare set as an additional information group. Preamble indexes included inthe additional information group (G3′) are for mobile stations in which1 bit of additional information of a preamble is active state indicationinformation. That is, the preamble indexes are for RRC_connected mobilestations that maintain an RRC connection with a base station.

The remaining preamble indexes except for the additional informationgroup G3′ are divided into a reserved allocation group G1 and acontention-based group G2. In this instance, the preamble indexes of thereserved allocation group G1 are allocated to a mobile stationperforming a handover, a mobile station that maintains uplink physicallayer synchronization and then receives notification of downlink servicestart, or a mobile station that receives allocation of an uplink radioresource for status reporting or measurement reporting from a basestation.

Meanwhile, the preamble indexes of the additional information group G3′allocated to the RRC_connected mobile station can be managed based oncontention. In this case, the mobile station randomly selects one fromamong the preamble indexes of the additional information group G3′ andtransmits it based on contention. The preamble indexes of thecontention-based group G2 are used when mobile stations having no RRCconnection with a corresponding base station, that is, mobile stationsbeing in an RRC_idle state, perform initial access.

According to the management of the preamble indexes, the base stationcan easily determine whether or not the mobile station performs initialaccess when additional information of a preamble transmitted from themobile station includes information indicating an idle state.Accordingly, a random access procedure performed after the mobilestation transmits the preamble may be classified and managed distinctly.

In the third exemplary embodiment, preamble radio resources may be alsodivided into a contention-based region and a reserved allocation regionaccording to an objective. In this case, if a mobile station generates apreamble based on a contention-based preamble index (e.g.,contention-based groups G2 of FIG. 7 and FIG. 8 and an additionalinformation group G3′ of FIG. 8), the mobile station transmits thepreamble through the contention-based region.

However, if a mobile station generates a preamble based on a reservedallocation based preamble index (e.g., reserved allocation groups G1 ofFIG. 7 and FIG. 8 and an additional information group G3 of FIG. 7), themobile station transmits the preamble through the reserved allocationregion. It is possible for the preamble radio resources to not bedivided into the contention based region and the reserved allocationregion, as in the first exemplary embodiment. In this case, mobilestations transmit a preamble through the same preamble radio resourcewith no regard to receiving allocation of a preamble index in advance.

Since a person of ordinary skill in the art can realize the method ofperforming random access based on preamble indexes that are managedaccording to the third exemplary embodiment of the present invention onthe basis of the first exemplary embodiment, a detailed description ofthe method of performing random access is omitted.

Mobile communication systems according to the exemplary embodiments ofthe present invention are based on OFDM(A), and radio resources arerepresented by two-dimensional coordinates with subcarrier indexes of afrequency axis and symbol indexes of a time axis. However, the presentinvention is not restricted to this, and can be applied to other radioresources of different types.

The above-described embodiments can be realized through a program forrealizing functions corresponding to the configuration of theembodiments or a recording medium for recording the program in additionto through the above-described device and/or method, which is easilyrealized by a person skilled in the art.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method of allocating a preamble in a mobile communication system, the method comprising: dividing preamble resources into a contention-based group and a reserved allocation group; and allocating a preamble resource of the reserved allocation group to a mobile station.
 2. The method of claim 1, further comprising: dividing preamble radio resources for preamble transmission into a contention-based region in which a preamble based on the preamble resource of the contention-based group is transmitted and a reserved allocation region in which a preamble based on the preamble resource of the reserved allocation based group is transmitted.
 3. The method of claim 1, further comprising broadcasting information on the preamble resources that are divided into the contention-based group and the reserved allocation group.
 4. A method of allocating a preamble in a mobile communication system, the method comprising: allocating one of preamble resources for constituting a preamble to a mobile station; and dividing preamble radio resources for preamble transmission into a contention-based region and a reserved allocation region in which the mobile station receiving the allocation of the preamble resource transmits a preamble.
 5. The method of claim 4, wherein the preamble resources are divided into a contention-based group and a reserved allocation group, and the step of allocating allocates a preamble resource of the reserved allocation group to the mobile station.
 6. The method of claim 2, wherein the mobile station transmits a preamble based on the allocated preamble resource through the reserved allocation region.
 7. The method of claim 2, wherein a mobile station not receiving allocation of the preamble resource randomly selects one from among available preamble resources and transmits a preamble based on the selected preamble resource through the contention-based region.
 8. The method of claim 1, wherein the step of allocating allocates the preamble resource to a mobile station satisfying a condition that is predetermined so as to allocate a preamble, wherein the predetermined condition is one of the following: a condition in which a mobile station in an active state does not maintain uplink physical layer synchronization with a base station and downlink data to be transmitted to the mobile station is generated; a condition in which a mobile station in an active state does not maintain uplink physical layer synchronization with a base station and the base station attempts to allocate an uplink radio resource for status reporting or measurement reporting to the mobile station; and a condition in which a mobile station located within a service area of a neighboring base station requests a handover.
 9. The method of claim 8, further comprising including a preamble resource to be allocated to a mobile station in information indicating downlink data generation when the downlink data to be transmitted to the mobile station is generated, and then transmitting the same.
 10. The method of claim 8, further comprising including a preamble resource to be allocated to a mobile station in a response message to a handover request when a neighboring base station requests a handover for the mobile station, and then transmitting the same to the neighboring base station.
 11. The method of claim 8, further comprising including a preamble resource to be allocated to a mobile station in uplink radio resource information when a base station attempts to allocate an uplink radio resource for status reporting or measurement reporting to the mobile station, and then transmitting the same to the mobile station.
 12. The method of claim 1, wherein when a preamble includes a first part representing a random property and a second part representing additional information, the step of dividing divides the preamble resources into a contention-based group and a reserved allocation group.
 13. The method of claim 12, wherein the step of allocating comprises: allocating a preamble resource of the additional information group to a mobile station that satisfies a condition corresponding to additional information among predetermined conditions for preamble allocation; and allocating a preamble resource of the reserved allocation group to a mobile station that satisfies the remaining predetermined conditions.
 14. The method of claim 12, wherein the additional information is one of the following: CQI (channel quality indicator) information representing whether a corresponding channel has greater quality than a predetermined value in a service area of a base station; information representing that a corresponding mobile station accesses through a handover process from a neighboring base station; and information representing that a corresponding mobile station does not perform initial access but maintains a connection with a base station.
 15. A method for a mobile station to perform random access in a mobile communication system, the method comprising: being provided with one of a plurality of preamble resources from a base station; generating a preamble based on the preamble resource; requesting random access by transmitting the preamble to the base station; and receiving a response to the random access from the base station and acquiring synchronization with the base station according to the response.
 16. The method of claim 15, wherein the preamble resources are divided into a contention-based group and a reserved allocation group, and the step of being provided with the preamble resource includes being provided with one of the reserved allocation groups from the base station.
 17. The method of claim 15, wherein the preamble resources are divided into a contention-based group, a reserved allocation group, and an additional information group when the preamble includes a first part representing a random property and a second part representing additional information, and the step of being provided with the preamble resource includes being provided with one of the additional information groups from the base station.
 18. The method of claim 15, wherein preamble radio resources are a contention-based region and a reserves allocation region for preamble transmission, and the step of requesting random access includes transmitting the preamble to the base station through the reserved allocation region.
 19. A method for a mobile station to perform random access in a handover process in a mobile communication system, the method comprising: receiving a message from a target base station in response to a handover request from a source base station, the message including a preamble resource that is selected by the target bases station from a plurality of preamble resources; generating a preamble based on the preamble resource; requesting random access by transmitting the preamble to the target base station; and receiving a response to the random access request and acquiring synchronization with the target base station according to the response.
 20. The method of claim 19, wherein the preamble resources are divided into a contention-based group, a reserved allocation group, and an additional information group when the preamble includes a first part representing a random property and a second part representing additional information, and one of the additional information groups is included in the response message. 